1. Field of the Invention
The present invention relates to a radiation detector using a scintillator, and more particularly, to a two-dimensional radiation detector which is referred to as a flat panel detector.
2. Description of the Related Art
In radiography which acquires an image by applying radiation to an object and detecting radiation which passes through the object, digital radiography (DR) which acquires an image by converting the detected radiation into an electric signal is popular. Generally, in DR, a flat panel detector (FPD) is used which includes a light receiving element having two-dimensionally arranged pixels and a scintillator layer placed on a light receiving surface of the light receiving element. Depending on the application, in most cases, a wide imaging area of several tens of centimeters or more per side is required for the FPD, and thus, the scintillator layer to be formed is required to have a large area. Therefore, the scintillator layer is formed by using vacuum deposition which enables formation of a large-area layer or an applying method of applying a binding agent having scintillator particles dispersed therein. In particular, a scintillator layer formed by vapor depositing cesium iodide has an advantage that, because cesium iodide is grown as needle crystals and so-called crosstalks are suppressed by a light guiding effect in the needle crystals, a high position resolution can be obtained. However, actually, adjacent needle crystals adhere to each other in places, and thus, in order to obtain a still higher position resolution, it is effective to cause the scintillator layer to have a greater extent of anisotropy in light propagation by causing the scintillator layer to have a structure in which two crystal phases having different refractive indices are completely separate from each other.
In order to manufacture such a scintillator layer including a structure in which two crystal phases having different refractive indices are completely separate from each other (phase separation structure), it is conceivable to employ a technology of micromachining a scintillator crystal, a technology of separating two phases of eutectic composition in one axial direction and growing the two phases, or the like. However, it is technically difficult to obtain by these technologies a phase separation structure having a large area of several tens of centimeters or more per side. In order to use a phase separation structure as a scintillator layer of an FPD, it is necessary to spread (tile) multiple phase separation structures processed to have a predetermined shape all over a surface of a light receiving element in order to secure a large imaging area. In this case, a problem is newly found that slight clearance which appears between adjacent phase separation structures due to limitations on the processing accuracy has a nonnegligible effect on a taken image. Specifically, a medium in the clearance which appears between phase separation structures is typically air (having a refractive index of 1.0), and thus, due to an effect of total reflection at an interface with a phase separation structure (tiling interface), the propagation characteristics of scintillation light generated in the phase separation structure locally change greatly. As a result, in pixels corresponding to the clearance between adjacent phase separation structures, the amount of incident scintillation light considerably reduces, and thus, the clearance between adjacent phase separation structures appears in the taken image as defects. Such defects are conspicuous when a large amount of scintillation light is generated at a location near a tiling interface. On the other hand, when a large amount of scintillation light is generated at a location far from the tiling interface, due to the great extent of anisotropy in light propagation of the phase separation structure, the amount of scintillation light which reaches the tiling interface is small, and thus, the defects are not conspicuous. In summary, when am object is actually imaged, bright portions and dark portions differ depending on each object, and thus, the effect of defects on the taken image differs accordingly. In order to perform calibration of such defects by image correction, a sophisticated correction technology is required with regard to each object.